This invention relates to an electron beam welding machine and more particularly to an electron beam welding machine which has a second beam adjusting means located between the electron gun with a first beam adjusting means and a workpiece.
In conventional electron beam welding machine, an electron gun is mounted on a welding chamber in which a workpiece is located and comprises a cathode, grid, anode, and a beam adjusting means including a focusing coil and a deflecting coil. A beam generator is contained in a vacuum chamber of the electron gun and the electron beams are directed towards the workpiece through an aperture in the wall of the welding chamber. Such a machine is disclosed, for example, in U.S. Pat. No. 3,617,686.
In such machines, it is generally necessary to evacuate the welding chamber and the electron beam gun chamber by means of a vacuum pump and to maintain them at high vacuum of about 1.times.10.sup.-4 Torr and about 1.times.10.sup.-5 to 1.times.10.sup.-6 Torr, respectively. However, gases contained in the workpiece such as H.sub.2 and O.sub.2 and the material of the workpiece itself converted into metal vapor are emitted from the surface of the workpiece during welding and eventually lower the vacuum in both vacuum chambers. If a large number of molecules enter the electron beam gun chamber the insulation effect of the vacuum in the proximity of anode and cathode is reduced, so that microdischarge and flash-over discharge occur. Microdischarge results in the formation of blow-holes at the weld zone while flash-over discharge leads to the formation of recesses of the surface beads, blow-holes and sags. If flash-over discharge occurs, furthermore, an excess current relay operates to stop operation of the apparatus in order to protect it.
These discharge phenomena occur frequently when the thickness of the workpiece exceeds 100 mm because the number of gas molecules and metal vapor molecules formed during welding increases with increasing thickness of the workpiece. When a 100 mm thick killed steel is welded, for example, microdischarge occurs 1 to 5 times within 10 minutes and the flash-over discharge occurs about once in 20 minutes. The vacuum in this instance is 1.times.10.sup.-3 Torr near the aperture in the wall of the welding chamber, 1.times.10.sup.-2 Torr near the workpiece and 10.sup.-5 to 10.sup.-6 Torr inside the electron beam gun chamber. Thus, the pressure ratio between the region near the aperture in the welding chamber and the electron beam gun chamber may be as large as 10.sup.2 -10.sup.3, so that the gas and metal molecules are liable to be sucked into the electron beam gun chamber. In consequence, molecules emitted from the surface of the workpiece enter the electron beam gun chamber and instantaneously reduce the vacuum near the anode and cathode down to 10.sup.-3 -10.sup.-4 Torr, thereby causing the discharge phenomena. Because of such discharge phenomena, it has been difficult in the conventional machines to weld, cut or bore a workpiece of a thickness of 100 mm or more by means of electron beams.
There is a prior art of an electron beam welding machine which has two beam adjusting means spaced from each other, which prior art is disclosed in GB patent specification No. 963,833.
The means, however are fixed so that the distance between the two means will not be changed. Therefore it seems that treatment of a thick workpiece of a thickness of 100 mm or more can not be effected satisfactorily.